1. Field of the Invention
The present invention relates in general to a cellular radio communication system supporting bandwidth scalability and, more particularly, to a method and an apparatus for transmitting and receiving a downlink signal with a guard band between adjacent sub-bands in the cellular radio communication system supporting bandwidth scalability.
2. Description of the Related Art
In these days, OFDM (Orthogonal Frequency Division Multiplexing) technology is being widely used for mobile communication systems.
OFDM technology has many advantages of removing interference between multipath signal components from a radio communication channel, guaranteeing the orthogonality among multiple-access users, and allowing an effective use of a frequency resource. Thereby OFDM technology is useful in a high-rate data transmission and a wideband system in comparison with DS-CDMA (Direct Sequence Code Division Multiple Access).
FIG. 1 is a diagram illustrating a frame structure of OFDM-based downlink.
Particularly, FIG. 1 shows a frame structure of OFDM-based downlink in EUTRA (Enhanced Universal Terrestrial Radio Access) which is the next generation mobile communication standard of 3GPP (3rd Generation Partnership Project).
Referring to FIG. 1, 20 MHz system bandwidth 101 contains one hundred resource blocks (RB) 102. A single RB is composed of twelve subcarriers 103 with frequency space of 15 kHz between adjacent subcarriers. There are fourteen OFDM symbol intervals 104, and a modulated symbol of a downlink channel is transmitted through each subcarrier 103 in each OFDM symbol interval 104. Each subcarrier section in each OFDM symbol interval is referred to as a resource element (RE) 106. As shown in FIG. 1, a single RB contains total one hundred sixty-eight REs (i.e., the product of fourteen OFDM symbols and twelve subcarriers). In a single OFDM symbol interval 104, one or more RBs may be allotted to transmission of a single downlink data channel, depending on data transfer rate.
FIG. 2 is a diagram illustrating a frequency domain mapping between a synchronization channel and a broadcasting channel according to system bandwidth in LTE system downlink.
One of important issues for offering a high-rate radio data service in a cellular radio communication service is to support scalable bandwidth. For example, the LTE (Long Term Evolution) system may have a variety of bandwidths such as 20 MHz, 15 MHz, 10 MHz, 5 MHz, 3 MHz, 1.4 MH, etc. Service providers may select one of such bandwidths to provide their services, and also user equipment may have various types such as a type for supporting bandwidths up to 20 MHz or a type for supporting 1.4 MHz bandwidth only. Additionally, the LTE-Advanced (hereinafter, referred to as LTE-A) system that has a goal to offer a service with a level required by the IMT-Advanced may provide a wideband service having 100 MHz bandwidth through carrier aggregation of LTE carriers.
Under a system based on scalable bandwidth, any user equipment that accesses initially the system has no information about system bandwidth and thus should be able to succeed in a cell search. Through this cell search, the user equipment may acquire a cell ID and synchronization between a transmitter and a receiver for demodulation of data and control information. System bandwidth may be obtained from a synchronization channel (hereinafter, referred to as SCH) during a cell search or obtained through demodulation of a broadcasting channel (hereinafter, referred to as BCH) after a cell search. The BCH is a channel for transmitting system information about a specific cell accessed by user equipment. After a cell search, user equipment demodulates the BCH before anything else. By receiving the BCH, user equipment may obtain cell information such as system bandwidth, an SFN (system Frame Number), and setting of some physical channels.
FIG. 2 exemplarily shows transmission of SCH and BCH according to system bandwidth. Use equipment performs a cell search through the SCH and, after a successful cell search, obtains system information about each cell through reception of the BCH.
In FIG. 2, a reference number 200 indicates the frequency axis. SCH 204 and BCH 206 are transmitted with 1.08 MHz bandwidth through a middle part of a system band, regardless of system bandwidth. Therefore, user equipment acquires an initial synchronization for a system by finding an RF carrier 202 regardless of system bandwidth and then performing a cell search for the SCH 204 in 1.08 MHz bandwidth around the RF carrier 202. After a cell search, user equipment obtains system information by demodulating the BCH 206 transmitted through the same 1.08 MHz bandwidth.
FIG. 3 is a diagram illustrating a transmission structure of SCH and BCH through a radio frame in the LTE system.
FIG. 3 shows transmission of SCH and BCH in a 10 ms radio frame 306. The SCH is divided into a primary synchronization signal (PSS) 300 and a secondary synchronization signal (SSS) 301 and transmitted at subframes #0 and #5. Each of the PSS 300 and the SSS 301 has one OFDM symbol interval 308 and is transmitted through 1.08 MHz bandwidth of a middle part in the entire system bandwidth 303 as shown in FIG. 2. The BCH 302 is transmitted using four OFDM symbol intervals at a subframe #0.
The LTE-A system requires a wideband for a higher-rate data transmission than the LTE system. Additionally, backward compatibility for LTE user equipment is also important, and LTE user equipment should be allowed to access the LTE-A system.
For the above, in downlink of the LTE-A system, the entire system band is divided into sub-bands with bandwidth allowing LTE user equipment to receive. LTE-A equipment available for higher receiving bandwidth may receive data through all sub-bands.
In case of the LTE-A system based on aggregation of LTE carriers, an effective solution to allow an OFDM transmitter of a base station to transmit signals in a useful band of LTE carrier by using only a single IFFT (Inverse Fast Fourier Transform) unit is required.